The conversion treatment of magnesium-containing metals is already known, for example, from JIS H-8651, MIL-M-3171, and so forth, and these conversion treatments have found practical application as underpaint coating treatments for magnesium-containing metals. However, all of these conversion treatment baths contain hexavalent chromium ions. Since hexavalent chromium ions are a pollution source, their presence is accompanied by a number of problems, for example, processing of the effluent from conversion treatment, management of the working environment, and the like.
These conversion treatments are also subject to other problems; for example, they require high treatment temperatures and long treatment times, and their treatment baths are very concentrated. Moreover, the conversion films produced by these conversion treatments have a strong tendency to be nonuniform.
Japanese Patent Publication Number Hei 3-6994 [6,994/1991] discloses a conversion treatment for magnesium-containing metals that does not use hexavalent chromium ions. This conversion treatment takes the form of a Cr.sup.6+ -free phosphate conversion treatment, which, however, is not sufficient to provide magnesium-containing metals with corrosion resistance. Specifically, after the phosphate treatment step, the overall treatment method of Japanese Patent Publication Number Hei 3-6994 teaches treatment with silicate and then silicone. The phosphate conversion coating by itself provides only a poor corrosion resistance and adherence when applied to the surface of magnesium-containing metals as an underpaint coating treatment. This treatment method also suffers from other problems; for example, it requires a multistep treatment process, high treatment temperatures, and long treatment times.
The use of treatment baths based on zinc phosphate, iron phosphate, zirconium phosphate, and so forth, is already known within the realm of phosphate-based conversion treatment methods, but these methods essentially can not provide the surface of magnesium-containing metals with a corrosion resistance satisfactory for practical applications.
A manganese phosphate treatment is reported in Section 7 of Japanese Industrial Standards ("JIS") H-8651, but this treatment bath is unsuitable for practical applications because it uses chromium, requires high treatment temperatures of 80.degree. C. to 90.degree. C., and requires long treatment times of 30 to 60 minutes.